1. Field of the Invention
The present invention relates to a C-type welding gun to be connected to a wrist of a robot, more particularly to a control system of a C-type welding gun which is driven by servomotors attached to the C-type welding gun.
2. Prior Art
There is known a C-type welding gun which is connected to a wrist of a robot and has a servomotor for driving the C-type welding gun (e.g. as disclosed in Japanese Utility Model Laid-Open Publication No. 5-18774). In this known C-type welding gun, when one ball screw of a movable arm moving mechanism connected to a shaft of the servomotor is rotated by the servomotor, the movable arm moving mechanism is reciprocated so that an electrode disposed in the movable arm moving mechanism is reciprocated. Another ball screw is rotated by the servomotor simultaneously with the rotation of one ball screw by way of a belt so that an equalizing mechanism is rotated, whereby another electrode of a C-shaped arm integrally connected to the equalizing mechanism is reciprocated. The moving amount of the electrode of the movable arm is detected by a moving amount detecting device. When the electrode of the movable arm is moved for a given amount, the rotation of the servomotor is stopped based on an instruction from a controller.
However, in the prior art C-type welding gun, since the weight of the gun is supported by a spring in the equalizing mechanism, the weight of the gun applied to the spring is varied depending on the posture of the gun. Accordingly, there are situations that the spring does not effectively operated depending on the posture of the gun, and hence the equalizing function can not be attained fully. Particularly, in case that the welding gun is directed upward when the posture thereof is varied 180 degrees opposite to the normal posture, the equalizing function cannot be attained at all. Since a single servomotor is stopped when the electrode of the movable arm is moved by a given length, there is a likelihood that there occur the difference of the distance between the electrode of the movable arm and the workpiece and the distance between the electrode of the C-shaped arm and the workpiece or the difference between the deformation of the electrode of the movable arm and that of the C-shaped arm due to the wear thereof depending on the thickness of the electrodes or the lapse of time. As a result, both electrodes do not reach the workpiece at the same time, whereby the electrode which reaches the workpiece first presses the workpiece, leading to the deformation of the workpiece.